Hyperthermia has proven to be useful in the treatment of malignant tumors. (JAMA 1976 235:2198; U.S. Pat. No. 3,991,770; U.S. Pat. No. Re. 32,066) Heat more than doubles the ability of chemotherapy to destroy malignant tumors and enhances the therapeutic index of chemotherapeutic agents. (J. Nat. Cancer Inst. 1982 68:487 Cancer Res. 1974 34:3117) Heat also enhances the treatment effect of ionizing radiation in treating tumors. (Applied Radiol. 1984 13:150 Cancer Res Suppl. 1984 44:471 4s) Although heat by itself is lethal to cancer cells, especially those in the anoxic tumor center, therapy with heat alone required that the tumor be rapidly raised to temperatures of 43 C. and remain at these temperatures for 2 to 3 hours. When tumors are slowly heated the cells become more thermoresistant and the killing effect of the heat is reduced in effectiveness. Because it is usually not possible to raise the whole body temperatures to 43 C. and maintain this temperature without serious and even fatal damage to normal cells, generalized body hyperthermia is less effective than localized hyperthermia and requires continuous observation and management by a team of several physicians. Rarely, is the systemic temperature raised higher than 42 C. At these temperatures serious coagulation defects in the blood occur. Other hazards to patients are malignant hyperthermia and brain damage. Generalized systemic hyperthermia requires a specialized team of physicians and is accompanied by serious risk. Therefore, most physicians prefer localized hyperthermia of the cancer itself.
The simplest method to induce such localized hyperthermia is by radiofrequency. Microwaves, which can easily be focused to the cancerous target are rapidly absorbed by passage through the intervening tissue. Absorption by body tissues reduces the amount of heat which can be generated in the tumor. Fifty percent of the microwaves are absorbed during the passage through the first 3 to 5 centimeters of tissue depending on the frequency. The power is so attenuated in the superficial tissues that microwaves cannot be successfully used to heat deep tumors. Therefore, longer wave lengths that are not as readily absorbed must be used to bring about temperature sufficiently high to treat deep tumors. However, a dilemma arises in that longer wavelengths cannot be focused and thereby be directed to the cancerous target. Previous experience has shown that power can be concentrated by an R.F. circuit which is properly tuned to the resonant frequency of a therapeutic RF generator at 13.56 mHz or 27.12 mHz. (U.S. Pat. No. 4,154,246) However, unless a tumor is located in a hollow viscus, it is necessary to surgically implant such resonant circuits which involves a major operative procedure for the patient. The present invention details a simple method for enhancement of RF heating not requiring surgery or the expense of tuned LC circuits.
In the past, iron needles have been used to concentrate heating of tissues. This was usually accomplished by placing stainless steel needles into the tumor mass and heating the needles by induction. The method entails the placement of multiple needles into the tumor and removing them after therapy. While such a procedure may be effective with superficial tumors it is not appropriate with deep tumors because of the long lengths of metal which extend out into the normal tissue and the difficulties involved in placing the needles. Also, needles cannot be allowed to extend from the lung across the plural space and exit from the skin. Such a procedure would bring about the leakage of air from the alveoli into the plural space and result in a serious condition known as pneumothorax. Large caliber needles would be required to provide adequate surface area for heating. Iron filings undergo corrosion in the tissues and cause necrosis. Of the various metals investigated in the form of a spheres (1/16 in. diameter) carbon steel has proved to be the most efficient followed by stainless steel. Nonetheless, these metals must be properly oriented in the energy field. If the metal in the form of a rod is parallel to the field, the maximum heating takes place but if it is perpendicular to the field no heating takes place at all. (J. Biomed. Materials Res. 1976 10:327) For all of these reasons, the introduction of a metal, even in powdered from, is generally unsatisfactory. Others have used pellets and seeds of copper and nickel but these must also be implanted. (Radiology 1986 154:243)